1. Technical Field
The present invention is directed to a nickel coating that is resistant to cracking and a process for forming such a nickel coating onto an article.
2. Art Background
Devices such as integrated circuits are mechanically and electrically connected to larger assemblies via lead frames. The integrated circuit or other device is mechanically mounted on the lead frame, and the device is then electrically connected to the leads on the lead frame. The lead frame is then electrically and mechanically connected to a larger assembly. After the device is mounted on the lead frame, the device is encapsulated for protection. The process for mounting and electrically interconnecting the device to the lead frame and connecting the lead frame to a larger assembly includes steps for deviceattachment, cure, wirebonding, encapsulation, trim-and-forming and soldering. Some of these steps subject the metal lead frame to mechanical stress and strain. For example, when the leads of the lead frame are formed according to the industrial standard, the forming angle is about 82 to about 90 degrees and the forming radius is about 250 xcexcm.
Lead frames have been formed from a variety of materials. Lead frame materials are selected for their mechanical strength, conductivity, machinability, formability, corrosion resistance, wirebondability, solderability, and thermal expansion. Although gold and palladium have the desired characteristics, the cost of these materials makes their use prohibitive for most applications. Copper and copper alloys also have many advantageous properties that make it suited for this application. A number of different copper alloys are used including alloy 151 (99.9 wt. % copper/0.1 wt. % zirconium); alloy 194 (97.5 wt. % copper/2.35 wt. % iron/0.03 wt. % phosphorous/0.12 wt. % zinc); and alloy 7025 (96.2 wt. % copper/3.0 wt. % nickel/0.65 wt. % silicon/0.15 wt. % magnesium). However, the corrosion of the copper in air and the difficulty of forming good soldered connections to copper create the need to use coated copper lead frames. The coating on the lead frame provides corrosion protection and provides a good solderable surface. An iron-nickel alloy, alloy 42 (58 wt % iron and 42 wt % nickel), also has properties that make it useful as a lead frame. However, the corrosion of this metal in air also precludes the use of uncoated alloy 42 as a lead frame material.
Typically, the copper and iron containing materials are coated with nickel to prevent the oxidation of the underlying copper or iron. However, nickel also oxidizes in air, and such oxides are undesirable. A thin layer of a metal that does not oxidize is plated over the nickel to prevent these oxides from forming. Examples of these materials, typically referred to as xe2x80x9cnoblexe2x80x9d metals, include silver, palladium, and gold. These thin coatings range in thickness from about 0.025 xcexcm to about 1.5 xcexcm.
Nickel coatings applied using conventional electrodeposition techniques have a tendency to crack when the lead frame is subjected to the stresses and strains associated with the trim-and-form steps that are discussed above. When the nickel layer cracks, the layer of noble metal thereon also cracks. When these metal coatings crack, the underlying copper or iron alloy oxidizes, corrodes, and migrates to the surface, in the presence of humidity. These surface deposits have an adverse effect on the packaged device. Consequently, a nickel coating for a lead frame that does not crack when the lead frame is subjected to stresses and strains associated with the packaging of electronic devices is required.
The present invention is directed to a conformable nickel coating that does not crack when a lead frame on which the conformable nickel coating is applied is formed according to the industrial standard, i.e. the forming angle of the leads is at least about 82 degrees with the lead frame with a forming radius of about 250 xcexcm. The forming angle 12 of the lead frame 10 and the forming radius 14 of the lead frame 10 are illustrated in FIG. 1. After forming, the lead frame is coupled to a device, packaged and placed into a larger assembly. The present invention is also directed to a process for forming a conformable nickel layer onto the surface of a metal substrate. Examples of metal substrates include copper substrates, copper alloy substrates, and iron alloy substrates such as iron-nickel alloys.
In the context of the present invention, a conformable nickel layer is a layer of nickel that conforms to the surface of the metal on which the nickel is plated in such a way as to resist cracking even when the coated lead frame is formed according to the standard described above. Forming a lead frame according to the above standard introduces deformations in the lead frame substrate. These deformations are in the form of surface undulations that have a depth of 0.1 xcexcm or more. Examples of sustrates that are susceptible to these surface deformations include alloy 151 substrates, alloy 194 substrates, alloy 7025 substrates, and alloy 42 substrates. The thicker the substrate, the more susceptible it is to cracking during forming. The conformable nickel coating of the present invention does not crack when the surface deformations that result from lead frame formation have a depth of less than 5 xcexcm. The thickness of the conformable nickel coating of the present invention is at least about 0.5 xcexcm. It is advantageous if the conformable nickel coating of the present invention has a thickness of at least about 1 xcexcm but less than about 25 xcexcm.
The conformable nickel coating of the present invention has an elongation of at least about 25 percent (measured using ASTM B489-85) when a substrate with the conformable nickel coating is formed according to the industrial standard. This is a substantial improvement over the elongation of prior art nickel coatings. Elongation (sometimes referred to as ductility) is a measure of the degree to which a coating deforms. The greater the elongation or ductility of the coating, the higher its resistance to cracking when the coated substrate is deformed.
In one embodiment of the present invention, a nickel layer that conforms to the substrate in the desired manner is obtained by plating the lead frame in a plating bath that contains about 75 g/l to about 130 g/l of nickel as a nickel complex such as Ni(NH2SO3)2 and about 3 to about 5 g/l of a nickel salt such as NiCl2xc2x76H2O. It is advantageous if the plating bath contains about 30 g/l to about 45 g/l of a buffer such as H3BO3 and about 5 ml/l to about 20 ml/l of a fluorochemical-containing wetting agent (e.g. an aqueous solution containing about 10 ppm of a fluorochemical such as fluorinated alkyl quaternary ammonium iodide). The composition of the bath is controlled so that the pH of the bath is maintained in the range of about 2 to about 2.5.
It is advantageous if the metal impurities in the bath are less than about 30 ppm. Metal impurities are any metals other than the nickel. In one embodiment of the present invention, the nickel is plated onto the metal lead frame substrate using a plating current density of about 5 Amps/dm2 to about 50 Amps/dm2 at a bath temperature of about 50xc2x0 C. to about 65xc2x0 C. It is advantageous if the bath is agitated at a speed of about 25 cm/sec to about 60 cm/sec during plating.
After the conformable nickel coating is formed on the lead frame substrate, a layer of a metal that does not oxidize in air is coated over the nickel to provide a solderable surface, because the nickel will oxidize in air and the oxidized nickel does not permit a good, solderable connection to be formed. Examples of such materials include gold, silver, palladium, and palladium alloys. These layers typically have a thickness of about 0.025 xcexcm to about 1.5 xcexcm and are formed on the nickel substrates according to processes well known to those skilled in the art. One exemplary method for forming a palladium layer onto a substrate is described in U.S. Pat. No. 4,911,799 to Abys et al, which is hereby incorporated by reference.